1 Answer by Expert Tutors

D=rt so 120 = xt for the first part of the problem. Therefore, t = 120/x. Using the increased speed, your new equation becomes 120 = (x + 1)(120/x - 1/2) because (x + 1) is the new speed and (120/x - 1/2) is the new time. Using your best algebra skills you can simplify this second equation eventually to x2 + x - 240 = 0 which factors into (x + 16)(x - 15) = 0. Solving this, x = -16 or x = 15 and you can throw out the first because it's negative. Therefore, x = 15km/h.